Should Time Machine created in Mac OS Extended be converted to APFS?

Note that Time Machine disks for Catalina cannot be APFS, they need to be HFS+.

Time Machine disks under Big Sur can be HFS+ or APFS, Apple says APFS is "preferred" for Time Machine under Big Sur.

https://support.apple.com/en-my/guide/mac-help/mh15139/10.15/mac/10.15

FWIW, Big Sur actually converts Time Machine backups to APFS. Hence, it's not necessary to manually do so.

It's not necessary.

dialabrain wrote:

FWIW, Big Sur actually converts Time Machine backups to APFS. Hence, it's not necessary to manually do so.

https://discussions.apple.com/content/attachment/f33ca747-5161-4206-a421-a2e2282b8af2

https://support.apple.com/en-my/guide/mac-help/mh15139/11.0/mac/11.0

[Big Sur section of Apple Instructions, link provided above]

"APFS or APFS Encrypted disks are the preferred format for a Time Machine backup disk. If you select a new backup disk that’s not already formatted as an APFS disk, you get the option to erase and reformat it. If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk."

steve626 wrote:

If the disk is a Mac OS Extended format disk that contains an existing Time Machine backup, you aren’t asked to erase and reformat the disk."

Correct. As I said, Big Sur automatically converts it. It doesn't ask. At least that's what it did with my five Macs running Big Sur. 😎

p.s. Specifically, APFS (case-sensitive).

I forgot to mention, Steve is correct in that Big Sur is the first and only version of macOS that supports Time Machine on APFS formatted drives.